Received signal to noise indicator

ABSTRACT

A method and device may be used to produce a received interference value in wireless communications. The device may include one or more components to measure a received power of a channel and measure an average noise plus interference power of the channel. The device may include a processor configured to calculate a received signal to noise indicator (RSNI) value. The RSNI may be based on the measured received power and the measured average noise plus interference power.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.13/371,582, filed on Feb. 13, 2012, which is a continuation of U.S.patent application Ser. No. 12/814,690, filed on Jun. 14, 2010, whichissued on Feb. 14, 2012 as U.S. Pat. No. 8,116,692, which is acontinuation of U.S. patent application Ser. No. 11/328,994, filed onJan. 10, 2006, which issued on Jun. 15, 2010 as U.S. Pat. No. 7,738,848,which is a continuation-in-part of U.S. patent application Ser. No.10/729,332, filed on Dec. 5, 2003, which claims priority from U.S.Provisional Application No. 60/440,073, filed on Jan. 14, 2003, whichare hereby incorporated by reference.

TECHNOLOGY FIELD

This application relates generally to wireless communications. Inparticular, the application relates to measurements with respect to suchcommunications.

BACKGROUND

This specification includes the following acronyms:

-   -   AP access point    -   BER bit error rate    -   CCK complementary code keying (RF modulation)    -   DSSS direct sequence spread spectrum    -   EIRP equivalent isotropically radiated power    -   ERP effective radiated power    -   FEC forward error correction    -   FER frame error rate    -   MIB management information base    -   OFDM orthogonal frequency division multiplexing    -   PBCC packet binary convolution coding    -   PHY physical layer    -   PLCP physical layer conversion protocol    -   PMD physical medium dependent    -   PPDU PLCP protocol data unit    -   PSK phase shift keying    -   PSNI perceived signal to noise indication    -   RPI received power indicator    -   RSSI received signal strength indicator    -   SQ signal quality    -   STA station

The current IEEE standard 802.11 is entrusted with the task of providinginterfaces, measurements, and mechanisms to support higher layerfunctions for efficient network management. Presently, the 802.11standard has defined several physical parameters, none of which iscompletely suitable for network management purposes. One example of ameasurable parameter is received signal strength indicator (RSSI), whichis a reportable parameter for each received frame but is not quantifiedin the standards, and is not fully specified. The standards do includecertain definitions in the context of RSSI, but it remains that RSSIposes certain limitations for use in network management since RSSIparameters from different stations (STAs) may not be uniformly definedand thus are not comparable.

A second suggested measurable parameter is the signal quality (SQ),which also happens to be an unquantized indicator of codesynchronization, but is only applicable to the DSSS PHY modulation andis not applicable to OFDM PHY modulations. Yet another measurableparameter is the RPI histogram, which, even though quantized andspecified, cannot make target measurements on any AP. RPI histogramsmeasure channel power from all sources including the 802.11 sources,radars, and all other interference sources, which is not helpful forrelying on the RPI histogram as a controlling parameter.

Current standards define received signal strength indication basedmainly on measurement of AP signals:

(1) on the same channel, same physical layer, and same station; and

(2) on different channels, same physical layer, and same station.

Significantly, measurements involving different physical layers and thesame or different stations, even though required, are not presentlyaddressed in the standards.

Network management needs comparative PHY measurements for use in handoffdecisions, for example. The following types of comparative PHYmeasurements are made.

1. To compare AP signals on the same channel, the same PHY, in the sameSTA.

2. To compare AP signals on the same channel, the same PHY, in differentSTAs.

3. To compare AP signals on different channels, the same PHY, in thesame STA.

4. To compare AP signals on different channels, the same PHY, indifferent STAs.

5. To compare AP signals on different PHYs in different STAs.

6. To compare AP signals on different PHYs in the same STA. Comparativemeasurements are crucial to handoff decisions for Network Management.

RSSI, as currently defined, only addresses categories (1) and (3) above.The RSSI is a measure of the RF energy received by the DSSS PHY or theOFDM PHY. RSSI indications of up to eight bits (256 levels) aresupported. The allowed values for RSSI range from 0 through RSSImaximum. This parameter is a measure by the PHY sublayers of the energyobserved at the antenna used to receive the current PPDU. RSSI ismeasured during the reception of the PLCP preamble. RSSI is intended tobe used in a relative manner, and it is a monotonically increasingfunction of the received power.

CCK, ER-PBCC: the 8-bit value of RSSI as described in 18.4.5.11.

ERP-OFDM, DSSS-OFDM, the 8 bit value is in the range of 0 to RSSImaximum as described in 17.2.3.2.

Some limitations of the RSSI indicator are: RSSI is a monotonic,relative indicator of power at the antenna connector, which indicatessum of desired signal, noise, and interference powers. In highinterference environments, RSSI is not an adequate indicator of desiredsignal quality. RSSI is not fully specified: there are no unitdefinitions and no performance requirements (accuracy, fidelity,testability). Since so little about RSSI is specified, it must beassumed that widely variant implementations already exist. It is notpossible to compare RSSIs from different products and perhaps not evenfrom different channels/bands within the same product.

Although RSSI has limited use for evaluating AP options within a givenPHY, it is not useful in comparing different PHYs. RSSI must be rescaledfor DSSS and OFDM PHYs. RSSI is clearly not useable by networkmanagement for load balancing or load shifting and RSSI from one STAdoes not relate to RSSI from any other STA.

SUMMARY

A method and device may be used to produce a received interference valuein wireless communications. The device may include one or morecomponents to measure a received power of a channel and measure anaverage noise plus interference power of the channel. The device mayinclude a processor configured to calculate a received signal to noiseindicator (RSNI) value. The RSNI may be based on the measured receivedpower and the measured average noise plus interference power.

BRIEF DESCRIPTION OF THE DRAWINGS

A more detailed understanding of the invention may be had from thefollowing description of preferred embodiments, given by way of exampleand to be understood in conjunction with the accompanying drawingswherein:

FIG. 1 shows the options for PHY measurements;

FIG. 1 a is a flow diagram showing a technique for deriving an input tothe FEC decoder;

FIG. 2 shows PSNI specified on BER curves; and

FIG. 3 shows example PSNI specification points.

FIG. 4 is a simplified block diagram of physical layer processing in areceiver.

FIG. 5 is a simplified block diagram of an embodiment for measuring asignal to noise indicator, such as a received signal to noise indicator(RSNI).

FIG. 6 is a flow diagram of RSNI calculation.

FIG. 7 is an illustration of a comparison of a RCPI measurements and theresulting forward error rate (FER).

FIG. 8 is an illustration of the signal to noise ratio (SNR) of RCPI andPSNI.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

It is desirable to provide a method of network management, consideringcomparative measurements of AP signals in all varying situationsincluding different physical layers and the same or different stations.

Described hereinafter is a demodulator-specific, subjective estimator ofperceived S/(N+I) specified by means of a quantized FER indication. Thefollowing is noted in the context of the description of the exemplaryembodiment.

All digital demodulators use tracking loops and complex post-processingto demodulate received symbols. Many internal demodulator parameters areproportional to perceived S/(N+I). Some examples are:

PSK: baseband phase jitter, base band Error Vector Magnitude (EVM)

DSSS: spreading code correlation quality

OFDM: frequency tracking and channel tracking stability

Demodulator internal parameters are available on a frame-by-frame basis.Demodulator parameters proportional to analog S/(N+I) are invariant withrespect to data rates. The same parameter may be used at any data rate.

Demodulator internal parameters may be specified and calibrated in acontrolled environment with respect to actual FER performance at two ormore operating points defined by rate, modulation, and FEC. Suchdemodulator internal parameters estimate FER performance in bothinterference environments and interference-free (noise only)environments and may be used as the basis for PSNI. For PSNI to be auseful indicator it is not necessary to specify which demodulatorinternal parameter to use as the basis for the indicator, but it issufficient to only specify how the quantized indicator relates to FER.

The following features are to be noted in connection with the inventiveuse of PSNI for network management:

PSNI is specified like RSSI as an 8-bit unsigned value, monotonicallyincreasing with increasing S/(N+I).

PSNI is logarithmically scaled to perceived S/(N+I). PSNI is based on ademodulator internal parameter which provides a fast estimator for FER.

Specify PSNI output indication across a range defined by two signalquality points: first point at a minimum useable signal quality level,second point at a maximum signal quality level.

Specify the output value and accuracy of the output value for at leasttwo FER points, and at least one FER point for each valid modulation,FEC, and data rate combination.

PSNI range may span the lower 40 db portion of the operating range ofS/(N+I) to cover high FERs at data rates from 1 to 54 Mbps, but higheror lower range spans may be used.

The PSNI indicator is a measure of the perceived, post-processingsignal-to-noise-plus-interference (S/(N+I)) ratio in the demodulator.The allowed values for the Perceived Signal to Noise Indicator (PSNI)parameter are in the range from 0 through 255 (i.e., eight binary bits).This parameter is a measure by the PHY sublayer of the perceived signalquality observed after RF downconversion, and is derived from internaldigital signal processing parameters of a demodulator used to receivethe current frame. PSNI is measured over the PLCP preamble and over theentire received frame. PSNI is intended to be used in a relative manner,and it is a monotonically increasing, logarithmic function of theobserved S/(N+I). PSNI accuracy and range are specified at a minimum oftwo different FER operating conditions. FIG. 3 supplies examplespecification points for a PSNI scaled to a 43 dB range.

FIG. 1 shows the options for PHY measurements, which can be used for aPSNI indicator. Referring to the receiver 10 in FIG. 1, the followinggeneral comments are valid for a wide range of modern modulation andcoding techniques. The signal to noise ratio at points A and B arenominally the same and may differ slightly due to added losses in theradio front end 12. The signal to noise ratio after the analog todigital conversion at A/D converter 14 is also nominally the same value,with minor additions to the noise associated with quantization error.

Therefore, in a high performance system, there is only a minordifference between the signal to noise ratio at point A and that at theinput to demodulator 16 and tracking loops. In a low complexity and lowperformance system, the signal to noise ratio difference between point Aand the input to demodulator 16 may be significant. The signal to noiseratio at the output of demodulator 16 (point C) is only indirectlyobservable by means of the bit error rate (BER). The BER at point Crelates to the signal to noise ratio at point B according to atheoretical demodulation performance curve which is adjusted to accountfor actual demodulator implementation losses.

Similarly, the BER at the output of FEC decoder 18 (point D) relates tothe FEC decoder input according to a theoretical FEC decoder performancecurve which is adjusted to account for actual FEC decoder implementationlosses. The frame error rate (FER) at point E at the output of the framecheck function 20 is a direct mathematical function of the BER and theerror distribution statistics at point D. There are normally noimplementation losses associated with the frame check. In general, forlow BERs, the FER is equal to the BER multiplied by the frame size inbits.

The frame check function 20 of receiver 10 in FIG. 1 may be implementedwith or without a frame parity check. In most practical designs, eachframe contains a parity check, which indicates (with high reliability)whether the block was received correctly or not. The most common paritycheck is a cyclic redundancy check (CRC), but other techniques arepossible and acceptable. If no frame parity check is used, the FER maybe estimated using a derived BER from the functioning of the FEC decoder18. Deriving the BER input from the FEC decoder 18 may be obtained usinga well known process, summarized as follows (see FIG. 1 a):

The output of the FEC decoder is generally correct. Therefore, thisoutput is obtained and stored (steps S1 and S2). The FEC encoding rulesare used to create a replica of the correct input bits (step S3) andeach bit is compared to the corresponding bit that was actually input tothe FEC decoder and stored (step S4). A count is increased for eachcomparison (step S5). Each disagreement (step S6) represents an inputbit error (step S7) which is accumulated. This derived BER (steps S9,S10) may then be used with the actual performance curve of the FECdecoder to estimate observed FER (step S11). The comparisons (error orno error—step S6) are continued until a count N is reached (step S8), atwhich time the count at step S7 is identified as the BER (step S9).

In this way, using the actual implementation losses with the theoreticalperformance curves allows one to relate the signal to noise measurementsat any point to the signal to noise measurement at any other point.

From a network management point of view, the signal quality delivered tothe user is best represented by the actual FER or observed FER (pointE). The PSNI concept provides an indicator which directly relates toobserved FER for all STAs, regardless of each STA's differentimplementation loss. This is accomplished by 1) basing the PSNI on themeasurement of an internal demodulator parameter, 2) specifying the PSNIindicator values with respect to observed FER at particular datarate/demodulation/FEC combination points, and 3) adjusting the internaldemodulator parameter measurement to account for actual FEC decoderlosses which occur downstream from the measurement point. By using ameasurement point internal to the demodulator, the measured signalquality already includes the effects of the STA front end losses. Byspecifying the PSNI indicator with respect to observed FER, actualdemodulator losses are included. By adjusting the demodulatormeasurement to account for actual FEC decoder losses, the validity ofthe indicator is preserved for all FEC decoders which the STA may use.

Since PSNI is based on an internal demodulator parameter, it can bemeasured and reported on a frame-by-frame basis. BER or FER measurementsat points C or E require thousands of frames for accurate measurement.Therefore PSNI is a practical, fast, and available indictor of observedsignal quality.

Measurements of analog signal to noise at points A or B can be performedquickly, yet without also knowing the sum of all the implementationlosses further downstream, they cannot be accurately related to observedFER at point E.

In these ways, the inventive use of PSNI for network management is morepractical to implement, faster to measure, requires no knowledge of STAimplementation, and is thus an improvement over the alternativesdiscussed here.

FIG. 2 shows PSNI specified on BER curves in the context of theinvention. FIG. 3 illustrates example specification points for a PSNIscaled to a 43 dB range.

The advantages of PSNI over RSSI include the following: The definitionof PSNI meets the requirements for RSSI in that the PSNI is an 8-bitunsigned value (for DSSS PHYs) and is proportional to received signalpower. PSNI may be reported in any data field calling for RSSI, whichmakes the PSNI indicator broadly applicable as an interlayer framequality measurement. PSNI MIB entries and reporting/posting may furtherbe mandated in 802.11 to make the PSNI improvements available to higherlayers.

The foregoing is a description of an exemplary embodiment of the PSNIindicator and method of network management. It is envisaged that theinvention is applicable to all modes of transmission including TDD, FDD,CDMA, and other modes without exception. It is also conceivable thatvariations of the described PSNI indicator and method with suitablemodifications are conceivable. All such modifications and variations areenvisaged to be within the purview of the invention.

FIG. 4 is a simplified block diagram of physical layer processing in areceiver. The receiver can be in a wireless transmit/receive unit (WTRU)or a base station. In the exemplary embodiment, the receiver is used inan IEEE 802 environment station, such as 802.11a, b, g or n station. Thefollowing description is in the context of the exemplary embodiment.However, in other embodiments, the receiver can apply to other wirelessstandards and to other receivers, such as in an access point orconverged technology device.

In FIG. 4, an antenna 900 receives a total radio frequency (RF) power.As illustrated at the antenna connector at point A, the total RF poweris a combination of the signal (S)+noise (N)+interference (I) from eachaccess point (AP) and the N+I from each channel. A radio front end 902processes this received signal and produces a baseband (BB) signal atpoint B. The quality of this signal can be represented as S/(N+I). Thebaseband signal is converted from analog to digital by an analog todigital converter (A/D). A demodulator and tracking loops 906demodulates the digital signal producing a demodulated signal. Theconfiguration of the demodulator and tracking loops 906 is based on thespecific physical layer protocol being used. Typically, the demodulatorand tracking loops 906 feedback gain values to the radio front end 902to control a gain of the automatic gain control.

At point C, the quality of the output of the demodulator and trackingloops 906 can be measured as a bit error rate (BER) at each data ratefrom each AP. For some physical layer implementations, a forward errorcorrection (FEC) decoder 908 is used. At point D, the quality of theoutput of the FEC 908 can be measured by the BER as well. A frame checkdevice 910 checks each frame of data for errors, such as by using acircular redundancy check (CRC). A quality at the output of the framecheck device 910 can be measured as the frame error rate (FER) at eachdata rate from each access point.

FIG. 5 is a simplified block diagram of an embodiment for measuring asignal to noise indicator, such as a received signal to noise indicator(RSNI). An antenna 912 receives the total RF power. At the antennaconnector 914, a received channel power measurement device 916 measuresthe received channel power indicator (RCPI). RCPI is a measurement ofthe RF signal, which includes noise and interference. Although themeasurement of the RCPI is shown as being at the antenna connector, inalternate embodiments, measurements at another point in the physicallayer processing are extrapolated to represent the RCPI at the antennaconnector 914. Accordingly, the RCPI, although measured by anothertechnique, is effectively a measurement at the antenna connector.

An average noise plus interference (ANPI) value is measured by an ANPImeasurement device 918 at the antenna connector 914. Althoughillustrated as being measured at the antenna connector, the ANPI may bemeasured at another point in the physical layer processing andextrapolated to represent the value at the antenna connector. The ANPIis a value representing the average noise plus interference power on themeasured channel. In one embodiment, ANPI is measured during themeasurement duration when NAV is equal to zero or, in other word, whenthe virtual carrier sense (CS) mechanism indicates an idle channel. Apreferred value for ANPI is defined in dbm using the same units as forRCPI. In one embodiment, ANPI may be derived as a summary metric fromthe noise histogram by calculating a weighted average of the histogrambin power levels. In an exemplary embodiment, ANPI may be calculated bya weighted average for the reported RPI densities assuming noise powerto be the mid range value for each of the nine defined ranges for theRPI levels, such as defined in Table 1, although other ANPI calculationtechniques may be used.

TABLE 1 RPI RPI Level (dBm) 0 RPI ≦ 92 1 −92 < RPI ≦ −87 2 −87 < RPI ≦−82 3 −82 < RPI ≦ −77 4 −77 < RPI ≦ −72 5 −72 < RPI ≦ −67 6 −67 < RPI ≦−62 7 −62 < RPI ≦ −57 8 −57 < RPI

In an 802.11 environment, it is difficult to derive a neutralinterference power measurement, due to the CSMA/CA and time-varyinginterference levels that other wireless systems typically do not have.By converting the interference power histograms into a scalar valueprovides a more meaningful useful measure of the interference level.

Using the measured RCPI and ANPI, a received signal to noise indicator(RSNI) is measured. The RSNI value is a received signal to noisemeasurement for a received frame at the antenna connector 914, such asthe currently in use antenna connector. This value is preferablyreported in an RSNI field, such as to an access point. The RSNI is addedto a beacon report, frame report and reassociation response. The RSNI isthe received signal to noise plus interference ratio derived from themeasured RCPI and ANPI. Preferably, the ANPI is the most recent ANPIvalue measured on the channel used to receive the frame; however, othertechniques may be used. An exemplary embodiment of a derivation of RSNIis a ratio of the received signal power (RCPI−ANPI) over the noise plusinterference power (ANPI), expressed in db, such as in half db steps.For a preferred range of −10 db to +118 db, RSNI=[(ratio in db)+10)*2].The ratio is (RCPI−ANPI)/ANPI. This ratio is preferably defined at themedium access control (MAC) layer.

FIG. 5 illustrates one embodiment of a circuit from calculating theratio for RSNI. A subtractor 920 subtracts the ANPI from the RCPI,producing RCPI−ANPI. This value is divided by the ANPI by a divider 922,producing the ratio for RSNI. This ratio is typically scaled to producethe RSNI value for the preferred range, such as to the preferred rangeof −10 dB to +118 db. Although the circuit in FIG. 5 is illustrated asbeing a subtractor 920 and a divider 922, other devices can be used toperform these calculations, such as a processor.

FIG. 6 is a flow diagram of RSNI calculation. In step 924, the RCPI iseffectively measured so as to represent the RCPI at the antennaconnector. In step 926, the ANPI is effectively measure so as torepresent the ANPI at the antenna connector. In step 928, RSNI iscalculated from the RCPI and ANPI.

In some implementations, RSNI can be used as an effective comparativetool to evaluate the delivered signal quality between stations. BecauseRSNI is measured at the antenna connector it provides a fair evaluationbetween stations. Various stations may have different RF/demodulationimplementations, which can skew the results downstream. Although FERmeasurements at point E in FIG. 4 can be used, that value cannot bemeasured frame by frame. FER can only be accurately measured over 100sto 1000s of frames. Also, FERs are comparable only at the same framesize and data rate. FIG. 7 is an illustration of a comparison of a RCPImeasurement at A and the resulting FER if the signal is kept to the sameobjective/subjective SNR for a Good, Medium and Marginal qualitystation. As illustrated on the left of the chart, all three stationshave the same RCPI and the same objective SNR. However, the actual FERsdiffer greatly based on the physical layer processing quality of thestation. On the right side, using the RSNI to maintain a same subjectiveSNR, the RCPI at A vary, but the FERs are equivalent for the variousstations. Accordingly, RSNI is a tool that may be used to betterallocate resources to stations of varying quality. FIG. 8 illustratesthe SNR relation of RCPI and PSNI in the demodulator.

What is claimed is:
 1. A method for use in a receiver for reporting areceived signal to noise ratio, the method comprising: calculating areceived signal to noise indicator (RSNI) value based on a measuredreceived power of a channel and a measured average noise plusinterference power of the channel, wherein calculating the RSNI valuefurther comprises: subtracting the measured average noise plusinterference power from the measured received power to determine adifference, and dividing the difference by the measured average noiseplus interference to determine the RSNI value; and reporting the RSNIvalue to an access point.
 2. The method of claim 1 wherein the RSNIvalue is included in a field of a beacon report.
 3. The method of claim1 wherein the RSNI value is included in a field of a frame report. 4.The method of claim 1 wherein the RSNI value is included in a field of areassociation response.
 5. The method of claim 1 wherein the receivedpower and the average noise plus interference power are measured at anantenna connector.
 6. The method of claim 1 further comprising: scalingthe RSNI value for a desired range.
 7. The method of claim 1 wherein themeasured received power is a measurement of a radio frequency (RF)signal that includes noise and interference.
 8. The method of claim 1wherein the RSNI value is in a range of −10 db to +118 db.
 9. The methodof claim 1 further comprising: transmitting the RSNI value in amanagement frame or a control frame.
 10. A receiver comprising: aprocessor configured to calculate a received signal to noise indicator(RSNI) value based on a measured received power of a channel and ameasured average noise plus interference power of the channel, whereinthe processor calculates the RSNI value by subtracting the measuredaverage noise plus interference power from the measured received powerto determine a difference, and dividing the difference by the measuredaverage noise plus interference.
 11. The receiver of claim 10 whereinthe processor is further configured to include the RSNI value in a fieldof a beacon report.
 12. The receiver of claim 10 wherein the processoris further configured to include the RSNI value in a field of a framereport.
 13. The receiver of claim 10 wherein the processor is furtherconfigured to include the RSNI value in a field of a reassociationresponse.
 14. The receiver of claim 10 wherein the received power andthe average noise plus interference power are measured at an antennaconnector.
 15. The receiver of claim 10 wherein the processor is furtherconfigured to scale the RSNI value for a desired range.
 16. The receiverof claim 10 wherein the measured received power is a measurement of aradio frequency (RF) signal that includes noise and interference. 17.The receiver of claim 10 wherein the receiver is a wirelesstransmit/receive unit (WTRU).
 18. The receiver of claim 10 wherein thereceiver is a base station.